1. Field of the Invention
The present invention relates to a composite ceramic board which has a plurality of ceramic insulating layers formed as a unitary structure upon firing, features a large strength, a high thermal conductivity, and is particularly suited as a wiring board for high-frequency use, relates to a method of producing the same, to an optical/electronic-mounted circuit substrate using said board, and to a mounted board equipped with said circuit substrate.
2. Description of the Related Art
Accompanying the trend toward highly densely integrating the semiconductor devices and transmitting signals at high frequencies in recent years, it has been urged to mount the semiconductor devices on a wiring board that has further improved thermal properties and electrical properties.
Namely, as the semiconductor devices are highly densely integrated, an increased amount of heat is generated from the semiconductor devices. To prevent the semiconductor devices from malfunctioning, it is necessary to quickly release the heat out of the devices. Therefore, the wiring board mounting the semiconductor devices must have a high thermal conductivity.
Further, transmitting the signals at a high frequency brings about an increase in the speed of the operation, while a delay in the signals hinders the attempt for increasing the speed of operation.
To prevent the delay in the signals, therefore, wiring layers must be formed using a low-resistance conductor having a small conduction loss.
Accompanying the recent widespread use of multi-media, on the other hand, it is becoming necessary to transmit and receive tremendous amounts of image data, and attention has been given to optical communication capable of transmitting and receiving large amounts of data at high speeds.
The optical communication device has a structure in which electronic semiconductor devices and multi-chip modules are connected together through optical waveguides in a complex manner frequently intersecting the waveguides. In order to decrease the size of the device by processing optical signals and electric signals using the same mounted substrate and to cope with the complex optical inter-connection, therefore, there has been frequently employed an optical/electronic-mounted circuit substrate of a structure forming optical waveguides on the ceramic board and mounting optical semiconductor devices and electronic semiconductor devices.
The optical/electronic-mounted circuit substrate, too, uses the above-mentioned wiring board, and must have a high heat-radiating property to cope with an increase in the amount of heat generated as a result of a high degree of integration and high frequencies, and must have a decreased resistance of the conductor to meet the demand for performing the operation at high speeds.
As a wiring board for mounting such semiconductor devices, there has heretofore been much used a ceramic board obtained by depositing conductor layers (wiring layers) of a high-melting point metal such as tungsten or molybdenum on the surfaces or inside of the insulating board made of alumina ceramics from the standpoint of reliability.
However, the above-mentioned conventional ceramic board is not capable of fully meeting the above-mentioned modern thermal requirements or electrical requirements.
For example, the conventional alumina ceramics substrate is satisfactory from the standpoint of thermal properties (thermal conductivity). However, since the wiring layers (conductor layers) have been formed of a high-melting point metal, the resistance can be lowered down to only about 8 mxcexa9/xe2x96xa1 or so. Therefore, signal insertion loss is very large, and favorable high-frequency characteristics are not obtained.
Besides, since the insulating board having a wiring layer serving as a terminal for receiving signals is formed of alumina ceramics having a high dielectric constant, high-frequency signals are reflected to a large degree and transmission characteristics are deteriorated.
In order to improve high-frequency signal transmission characteristics by suppressing the reflection of signals, there has been proposed a wiring board obtained by using glass ceramics having a low dielectric constant as an insulating layer of the signal input portions, which is formed integrally with a reinforced glass (Japanese Unexamined Patent Publication (Kokai) No. 239394/1991).
This wiring substrate is satisfactory from the standpoint or high-frequency characteristics (electric characteristics) since the insulating layer mode of glass ceramic has a low dielectric constant and a conductor layer is formed by co-firing a low-resistance conductor such as of copper.
However, the thermal conductivity of the glass ceramics is several watts/mxe2x80xa2K at the greatest. Therefore, though high-frequency characteristics can be accomplished, heat is not smoothly radiated from the semiconductor device, and thermal properties (thermal conductivity) are not satisfactory causing the device itself to malfunction.
Besides, the insulating board has a small resistance and is cracked at the time of mounting various devices.
A variety of proposals have heretofore been made to improve problems related to the above-mentioned thermal properties and electric properties (high-frequency characteristics) of the conventional substrate.
For example, Japanese Unexamined Patent Publications (Kokai) Nos. 15101/1995 and 151045/2000 disclose wiring boards formed by co-firing an insulating board made of aluminum oxide and a conductor layer of copper or of a combination of copper and tungsten or molybdenum.
Further, Japanese Unexamined Patent Publications (Kokai) Nos. 106880/1998, 214745/1998 and Japanese Patent No. 3061282 disclose wiring boards equipped with an insulating board having a plurality of insulating layers of dissimilar dielectric constants that are formed integrally together.
According to Japanese Unexamined Patent Publication (Kokai) No. 15101/1995, however, all of the wiring layers (conductor layers) are arranged in the insulating board and are co-fired simultaneously, the insulating layer on the surface of the insulating board is removed by polishing so that the inner wiring layer is exposed on the surface of the insulating board, or a thick-film method or a thin-film method is applied onto the surface of the wiring board after firing thereby to form a surface wiring layer (surface conductor layer).
Therefore, a polishing step, a thick film-forming step and a thin film-forming step are indispensable for forming the surface wiring layer accompanied by such problems as an increased number of the production steps, a decreased yield and an increased cost.
According to Japanese Unexamined Patent Publication (Kokai) No. 151045/2000, the firing is conducted at a temperature of not higher than 1500xc2x0 C. and, hence, low-melting point metals such as copper and the like are separated little, and a conductor layer having a small resistance is formed.
Besides, the surface wiring layer (conductor layer) of the insulating board, too, is formed by co-firing making it possible to avoid an increase in the cost of production.
However, the insulating layer which is formed of alumina has a dielectric constant of as high as about 9.
In this case, the loss due to the reflection of input signals increases in a region where the signals have a frequency of about 40 GHz, resulting in a decrease in the characteristics.
This holds true for the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 15101/1995.
According to Japanese Unexamined Patent Publications (Kokai) Nos. 106880/1998, 214745/1998 and Japanese Patent No. 3061282, further, the layers of low dielectric constant are formed as a unitary structure and the insulating board is formed of glass ceramics of a composition that can be fired at a low temperature. Therefore, signals of high frequencies can be processed by using a wiring layer (conductor layer) formed of Cu, Au, Ag or Pt having a low resistance as a chief component.
The boards, however, are not satisfactory in regard to the strength since the insulating board is made of glass ceramics.
Even when, for example, a reinforced glass is used, the bending strength is about 200 MPa at the greatest.
Problem further arouses concerning the heat-radiating property (thermal conductivity).
It is therefore a first object of the present invention to provide a composite ceramic board which has a large strength, a high thermal conductivity, exhibits excellent high-frequency characteristics, and is particularly useful as a wiring board for high-frequency use.
A second object of the present invention is to provide a method of producing the above composite ceramic board.
A third object of the present invention is to provide an optical/electronic-mounted circuit substrate equipped with the above composite ceramic board.
A fourth object of the present invention is to provide a mounted board mounting the above-mentioned optical/electronic-mounted circuit substrate on an electronic circuit formed on the surface of the mother board through external connection terminals.
According to the present invention, there is provided a composite ceramic board wherein insulating layers of alumina ceramics and dielectric layers of ceramics having a dielectric constant smaller than that of said insulating layers, are laminated as a unitary structure, and conductor layers containing at least one kind of low-resistance conductor selected from the group consisting of Au, Ag, Cu and Pt are formed on the surfaces and/or in the inside.
In the composite ceramic board of the present invention, an insulating board is formed by the insulating layers of alumina ceramics having a high dielectric constant and insulating layers of ceramics having a low dielectric constant which are integrally formed together by firing, to exhibit excellent properties inherent in these insulating layers.
That is, the composite ceramic board includes layers of alumina ceramics that exhibit a high thermal conductivity and large strength and further includes ceramic layers having a low dielectric constant. Upon providing the low-dielectric ceramic layers with an electrode for receiving external signals, therefore, the loss due to the reflection of input signals is effectively decreased and good high-frequency signal transmission characteristics are accomplished.
Further, the above insulating board is formed as a unitary structure by firing at 1200 to 1500xc2x0 C. Therefore, the conducting layer of a low-resistance conductor such as of copper is formed by co-firing offering a great advantage from the standpoint of production steps.
Moreover, since the conductor layer is formed of a low-resistance conductor such as of copper, the resistance can be decreased and conduction loss can be decreased offering a great advantage in transmitting the high-frequency signals.
In the composite ceramic board of the present invention, when the insulating layers and/or the dielectric layers comprise laminates, wiring can be arranged inside the laminates, and a plurality of devices such as semiconductors can be mounted being highly densely integrated.
Further, the dielectric layer is formed at a position to be exposed on the surface of the board, and a conductor layer that serves as an electrode for receiving external signals is formed on the exposed portion. Namely, the conductor layer that serves as a terminal for receiving signals is formed on the low-dielectric insulating layer to lower the reflection of high-frequency signals input from the wiring layer thereby to avoid a decrease in the transmission characteristics.
In order to effectively conduct the co-firing with the conductor layer containing a low-resistance conductor such as of copper in the present invention, furthermore, it is desired that the alumina ceramic insulating layer contains silica (SiO2) and Mn2O3 in addition to alumina (Al2O3) which is a chief component and, particularly, contains from 2 to 15% by weight of manganese oxide and from 2 to 15% by weight of silicon oxide.
It is further desired that the low-dielectric ceramic layer contains, as a chief component, at least one of those selected from the group consisting of mullite, forsterite, enstatite, silica and cordierite for it makes easy to cope with higher frequencies.
It is desired that the conductor layer has a sheet resistance of not larger than 8 mxcexa9/xe2x96xa1 calculated as having a thickness of 15 xcexcm and, further, contains at least one kind of a high-melting point metal selected from W and Mo in addition to a low-resistance conductor and, particularly, contains the low-resistance conductor in an amount of from 10 to 70% by volume and contains the high-melting point metal in an amount of from 30 to 90% by volume.
This makes it easy to decrease the resistance of the conductor layer formed on the surface or in the via-holes of the alumina insulating layer.
As a particularly preferred embodiment of the invention, further, there is provided a composite ceramic board wherein the dielectric layer contains forsterite and cordierite an chief crystal phases and, further, contains, as sub-components, at least one of SiO2, Zn, Mn and alkaline earth metals and/or non-loadxe2x80xa2non-alkaline borosilicate glass in an amount of from 0.1 to 20% by weight per the whole amount.
the composite ceramic substrate of the above embodiment features a particularly highly intimate adhesion between the alumina insulating layer and the dielectric layer, without warping or cracks of the board, and exhibits very large strength and thermal conductivity as compared to those of the glass ceramics.
In particular, upon containing SiO2 as a subcomponent, the firing can be conducted at a lower temperature, which is desirable.
Upon setting the composition of the dielectric layer having a low dielectric constant to be as described above, it is allowed to decrease the loss due to the reflection of input signals in a high-frequency region and, particularly, to decrease the loss in a region of about 60 GHs.
It is particularly desired that the dielectric layer contains cordierite in an amount of from 20 to 40% by weight per the whole amount.
This decreases the difference in the coefficient of thermal expansion from the alumina insulating layer, effectively suppresses the occurrence of pealing from the alumina insulating layer and of cracks, suppresses the dielectric constant of the low-dielectric ceramic insulating layer to be not larger than 6 to thereby effectively decrease the signal loss.
It is further desired that the alumina insulating layer has a bending strength of not smaller than 350 MPa.
This prevents the board from being cracked when the devices are being automatically mounted and prevents a drop in the yield.
It is further desired that the alumina insulating layer contains Mn in an amount of from 2 to 15% by weight calculated as an oxide thereof, contains Si in an amount of from 2 to 15% by weight calculated as an oxide thereof, contains at least one of Mg, Ca, D, Nb, Cr and Co in an amount of from 0.1 to 4% by weight calculated as an oxide thereof, and has a relative density of not smaller than 95%.
This makes it easy to maintain the strength and the thermal conductivity of the alumina insulating layer.
According to the present invention, further, there is provided a method of producing a composite ceramic board of the above-mentioned preferred embodiment by applying an electrically conducting paste onto low-dielectric green sheets and onto alumina green sheets containing an oxide powder that contains at least one of Sn, Mn, and alkaline earth metals and/or non-leadxe2x80xa2non-alkali borosilicate glass powder for the forsterite powder and the cordierite powder in an amount of 0.1 to 10% by weight per the whole amount, laminating the low-dielectric green sheets and the alumina green sheets, and firing the obtained laminate at 1200 to 1500xc2x0 C.
According to this method, the alumina ceramics having excellent strength and thermal conductivity and the dielectric layer having a low dielectric constant are fired simultaneously, and conductor layers are easily formed inside the substrate and on the surfaces of the substrate.
Further, prior to laminating the low-dielectric green sheets and the alumina green sheets, via-holes are formed in the low-dielectric green sheets and/or in the alumina green sheets, and the via-holes are filled with an electrically conducting paste.
Thus, a three-dimensional wiring is formed in the ceramics, the multi-layer ceramic substrate is easily realized in a small size incorporating such functions as capacitors and inductors.
It is further desired to prepare the low-dielectric green sheets by adding the cordierite powder in an amount of from 20 to 40% by weight per the whole amount.
This makes it possible to match the coefficient of thermal expansion with that of the alumina green sheets, and to reduce the warping and cracks during the firing.
It is further desired that 2 to 15% by weight of Mn2O3, Z to 15% by weight of SiO2, 0.1 to 4% by weight of at least one of MgO, CaO, B2O5, Nb2O5, Cr2O3 and CoO3 and the remainder of alumina power are mixed together, and are molded to prepare an alumina green sheet.
This makes it easy to lower the firing temperature while nearly maintaining the strength and thermal conductivity of the alumina insulating layer, and to improve the yield of products.
It is further desired to prepare the electrically conducting paste by mixing a copper powder in an amount of from 10 to 70% by volume, and a tungsten powder and/or a molybdenum powder in an amount of from 30 to 90% by volume. This makes it easy to form a conductor layer of a low resistance even at a firing temperature of from 1200 to 1500xc2x0 C., which is higher than the melting point point of copper.
According to the present invention, further, there is provided an optical/electronic-mounted circuit substrate comprising a composite ceramic board of the invention, an optical waveguide and an optical semiconductor device mounted on one surface side of the composite ceramic board, an electronic semiconductor device mounted on one surface or on the other surface of the composite ceramic board, and an external connection terminal provided on the dielectric layer of the composite ceramic board.
The optical/electronic-mounted circuit substrate of the invention realizes the optical/electronic-mounted circuit substrate of a small loss in a portion of receiving high-frequency signals.
It is particularly desired that the insulating layers and/or the dielectric layers of the composite ceramic board of the invention used in the above-mentioned optical/electronic-mounted circuit substrate, comprise laminates.
Therefore, even when a plurality of semiconductors are mounted, the wirings can be arranged even inside the laminate making it easy to accomplish a highly dense mounting.
It is further desired that the electronic semiconductor device and the optical semiconductor device are mounted on the opposing surfaces of the insulating board.
This makes is possible to decrease the size and to improve the reliability.
It is further desired that the electronic semiconductor device is contained in a cavity formed in the surface of the insulating substrate, and the cavity is air-tightly sealed with a cap.
This stabilizes the characteristics of the electronic semiconductor device and improves the reliability thereof.
It is further desired that the electric layer of the insulating board is formed on a portion of the surface of the insulating layer.
This enables a part generating large amounts of heat, such as an electronic semiconductor device to be mounted on an insulating layer having a high thermal conductivity provided with the dielectric layer.
It is further desired that the optical semiconductor device is provided inside the optical waveguide.
This stabilizes the characteristics of the optical semiconductor device and improves the reliability thereof.
The invention further provides a mounted board wherein an electronic circuit including capacitors, resistors and wiring conductors is formed on the surface of a mother board, an optical/electronic-mounted circuit substrate of the invention is mounted on the electronic circuit via external connection terminals, and a reflection loss of when high-frequency signals of 40 GHz are input to the optical/electronic-mounted circuit substrate is not larger than xe2x88x9210.0 dB.
This mounted board makes it possible to realize an optical communication which is capable of transmitting and receiving large amounts of data at high speeds.